2. Field of the Invention
This invention relates to the automatic identification, selection, and changing of font elements in a serial printing apparatus which utilizes rotary printing elements having a central hub supporting resilient spokes which carry type slugs.
2. Description of the Prior Art
The prior art contains many examples of type element changing devices. Such devices exist in computer-controlled machine tools, in the printing industry and in photo-composition machines, as well as in typewriters. Prior art changing mechanisms utilized with typewriters and printers, such as for example those described in U.S. Pat. Nos. 3,645,372, 3,892,303 and 4,088,217, are slow, mechanically complex and not suitable for incorporation into the present rotary printing element serial printers used in word processors, computer output terminals and intelligent typewriters. This general type of serial printer is commonly referred to as a "daisy wheel" or cup wheel printer (depending on the shape of the print element) and is generally described in U.S. Pat. Nos. 3,954,163, 4,091,911, 4,118,129, 4,126,400, 4,127,336, 4,147,438 and 4,149,808.
A prior art serial printer utilizing a changing mechanism for rotary print wheels, such as that disclosed in U.S. Pat. No. 4,026,403, is limited to exchanging print wheel elements in a known predetermined sequence and in addition uses a complex exchange mechanism or arm that transfers the print wheel elements between the printing station and the storage location, thus requiring two exchanges of the print wheel element for each transfer of the print element.
The present invention overcomes the prior art restrictions by providing a mechanically simple changing mechanism which can rapidly and efficiently change print wheel type elements directly between storage and print carriage in a single exchange under microprocessor control.
A typical example of the requirements for a rapid computer-controlled element changing mechanism in a daisy wheel serial printer is when the text of a scientific document requires the use of Roman type font as well as italics and mathematical symbols, all of which cannot be placed on a single print wheel, which typically has from 88 to 125 characters. Another example is in printing multi-language text such as Japanese, Russian, and English. Thus, in order to appropriately print these types of documents, several exchanges of print wheels is required. Indeed, the rapid speed and the flexibility of prior art rotary print wheel printers are partially defeated by the need to continually stop the printer so an operator can manually change the type elements.